Much of the electrical power used in homes and businesses throughout the world is produced in power plants that burn a fossil fuel (i.e. coal, oil, or gas) in a boiler. The resulting hot exhaust gas (also sometimes termed xe2x80x9cflue gasxe2x80x9d) turns a gas turbine or boils water to produce steam, which turns a steam turbine, and the turbine cooperates with a generator to produce electrical power. To achieve good thermal efficiency for the power plant, the hot exhaust gas flows from the boiler through a heat exchanger (also sometimes termed an xe2x80x9cair preheaterxe2x80x9d) in which the air input flow to the boiler is preheated. The partially cooled exhaust gas is directed to the exhaust stack.
An important consideration for modern power plants is the cleanup of the exhaust gas. The exhaust gas produced in the boiler contains gaseous pollutants such as nitrogen oxides (xe2x80x9cNOxxe2x80x9d) and sulfur oxides (xe2x80x9cSOxxe2x80x9d), as well as particulates termed xe2x80x9cfly ashxe2x80x9d. Environmental laws establish permissible levels of gaseous pollutants and particulates that may be emitted from the exhaust stack of the plant. Various types of pollution-control equipment are therefore available to reduce the levels of gaseous pollutants and particulates from the exhaust gas before it reaches the exhaust stack. For example, among other methods, NOx is often removed by selective catalytic reduction (SCR) and/or selective non-catalytic reduction SNCR, and fly ash is often removed by an electrostatic precipitator (ESP) and/or a baghouse. The invention herein deals with those particular pollution control systems which utilize ammonia within the process in order to initiate, cause and/or supplement the process.
SCR is the catalyst mediated reduction of NOx with ammonia and provides the highest NOx removal efficiency of all NOx control technologies. In this process, ammonia is injected into the flue gas as a reagent for reducing NOx. On the catalyst surface absorbed ammonia reacts with NOx to form molecular nitrogen and water vapor:
4NH3+4NO+O2xe2x86x92CAT4N2+6H2O
8NH3+6NO2xe2x86x92CAT7N2+12H2O
NOx reduction efficiencies of up to 95% are possible with a properly tuned and sized conventional SCR system on a boiler and, in this regard, in many situations, SCR is often the preferred and/or the only practical technology available to provide compliance within the parameters established by the applicable emission control laws. Other SCR bases NOx reduction technologies of the type described above are used in so-called staged systems (i.e. SNCR used in conjunction with in-duct SCR and/or air preheaters having catalyzed heat transfer elements) and SCR for combined and simple cycle gas turbine applications. In all such instances, a key ingredient in the system operation is the safe, economic and reliable availability of ammonia
Other pollution control systems used in power plants, and heavy industry applications, which require the heavy utilization of ammonia, include: injection of ammonia for flue gas conditioning to assist in the removal of fly ash (see U.S. Pat. Nos. 4.064,219, 5,034,030, and 5,567,226); ammonia injection in situations where the fuel is a high sulfur content coal or oil which cause a so called xe2x80x9cblue plumexe2x80x9d because of excessive SO3 (in such situations the ammonia usage reduces the blue plume by removing excessive SO3 by formation of ammonium sulfate and bisulfate, see U.S. Pat. No. 5,024,171); ammonia based SNCR systems (see U.S. Pat. Nos. 3,900,554, 4,507,269, and 4,636,370); and in conjunction with SCR systems applied in combined cycle combustion turbines. Once again, a key driving ingredient in all such systems is the safe, economic and reliable availability of ammonia.
Ammonia for uses such as described above, are generally delivered to power plants in the form of anhydrous ammonia, or aqueous ammonia. Anhydrous Ammonia is used in massive quantities world-wide for many industrial and agricultural purposes. Ammonia is gas at ambient temperatures and pressures, and is normally shipped and stored as a liquid, either in pressure vessels at ambient temperature, and high pressure (i.e. over 16 bars), or in refrigerated vessels at ambient or nearly ambient pressure, and at about xe2x88x9233xc2x0 C. It is transported in bulk in ships, barges, and railroad tank cars, and in tank trucks on public roads and highways. It is frequently stored in large quantities at industrial sites in populated areas and is frequently used as the working fluid in large refrigeration systems. It is now coming into wider use for the removal of NOx from flue gas at power generating stations in urban areas.
Anhydrous ammonia is an extremely hazardous, toxic, and volatile material. In the event of an accidental discharge, it can cause immediate death to humans and animals and rapid death to trees and plants. Both anhydrous liquid ammonia, and concentrated aqueous liquid ammonia, display a deadly characteristic which substantially increases the risk of widespread injury and death in case of a spill. Specifically, upon sudden release to the atmosphere, as might occur in a sudden and accidental discharge (i.e. a storage failure at a power plant, a train wreck, or a traffic accident), the ammonia forms a cloud produced of an aerosol fog of liquid ammonia droplets. Unlike gaseous ammonia, which, though toxic, is lighter than air and quickly dissipates to harmless concentrations, the cloud can persist for a surprisingly long time, as long as several hours, before it finally disappears. The cloud is typically heavier than air and tends to drift along the surface of the earth, i.e., the ground or the surface of a body of water. The cloud moves with the wind and can sweep over a total area, i.e., a xe2x80x9cfootprint,xe2x80x9d much larger than the area covered by the cloud at any one moment. Contact with the cloud is instantly incapacitating, and a single breath can be fatal. Substantial numbers of bulk shipments of anhydrous ammonia routinely move through or near densely populated areas. It is estimated that an anhydrous ammonia spill from a 40,000 pound truck trailer would generate a cloud having an average lethal footprint of 29 acres, that is, an area of 29 acres in which the concentration of ammonia would reach a lethal level, about 0.5 percent, before the cloud eventually dissipated. Although this is an extreme example, it is interesting to note that a large SCR installation with ammonia demands of 3,000 pounds per hour, and a 5 day supply of ammonia reserve supply, will require on-site storage of approximately 360,000 pounds of this very toxic, volatile, and difficult to handle chemical.
In addition to the inherent danger of storing, transporting and handling large quantities of ammonia, the expense insofar as safety aspects, insurance costs, specialized training, and the difficult to quantify emotional exposure of living and working next to a such potential catastrophe, it is apparent that if another, less hazardous commodity could be transported instead of ammonia, and then be readily converted back to ammonia, the hazards associated with ammonia shipment and handling would be considerably reduced. To some extent, attempts have been made in the supply of ammonia for NOx control in power plant environments by substituting concentrated aqueous liquid ammonia for anhydrous ammonia. Such a solution has achieved only limited success, due to any number of factors, for example: the high energy cost of vaporizing the water carrier, relatively costly storage facilities; and, even though aqueous ammonia is safer to handle than anhydrous ammonia, it is still very difficult and costly to handle in a safe manner.
Urea is an ideal candidate as an ammonia substitute. Urea is a non-toxic chemical compound and, for purposes of this discussion, presents essentially no danger to the environment, animals, plant life and human beings. It is solid under ambient temperatures and pressures. Consequently, urea can be safely and inexpensively shipped in bulk and stored for long periods of time until it is converted into ammonia. It will not leak, explode, be a source of toxic fumes, require pressurization, increase insurance premiums, require extensive safety programs, or be a concern to the plant, community and individuals who may be aware of the transportation and/or storage dangers of ammonia. Furthermore, by means of the present invention, urea can be used to produce gaseous ammonia:
on-site
on-demand
with rapid response time
with maximum turn down availability
with utmost safety
with significant economies
with automatic operation
with low maintenance
The present invention fulfills this very serious need for providing a safe, economical and dependable supply to fossil fuel burning facilities, such as power plants, to assist in the control of NOx, SO3 and particulate pollutants therefrom.
The invention herein teaches the production of ammonia on-site and on-demand from a safe, benign urea feed stock. A rich urea solution is fed to a hydrolyzer assembly. The hydrolyzer assembly is operative to produce a gaseous mixture containing ammonia and CO2 and water vapor, as well as a lean solution of urea. The lean urea solution is recycled (a more or less closed loop system insofar as the urea solution), for intermixing with the rich urea solution, followed by the hydrolysis step, and so on. The ammonia containing gaseous mixture is drawn from the hydrolyzer as required for use in the various pollution control systems. Because the lean urea solution is recycled within this very closed system, there is no waste to content with. Furthermore, because the pollution control systems for which the ammonia is required are from large fossil fuel burning sources, emitting flue gas which already contains very large quantities of CO2 and H2O, the relatively small additional quantities of CO2 and H2O present in the gaseous mixture produced by the urea hydrolysis, are of no significance in overall scheme of things.
The search for a safer, simpler, and less expensive alternative to ammonia has been going on for quite some time. Indeed, there have even been attempts to use a urea feedstock for ammonia requirements in fossil fuel burning applications, such as in electrical generation facilities. In this regard, the reader is referred to U.S. Pat. Nos. 4,220,635, 5,252,308, 5,281,403, 5,240,688, 5,399,325, 5,543,123, and Japanese Patent No. HEI 2-191528, all of which addressed the considering urea as a feedstock for conversion to ammonia. Nevertheless, all of these prior attempts suffered from one or more very serious deficiencies, for example: some prior systems depended on expensive and yet to be proven catalysts to promote the conversion of urea to ammonia; most of the prior attempts were single pass systems, which could produce large quantities of waste which had to be further processed, or otherwise required significant steps and/or energy to operate the system; some of the systems produced other chemical contaminants which could not be discharged into the atmosphere without treatment, or, if injected into the flue gas, might result in catalyst poisoning and/or secondary pollutants; and the like.
In addition to the above patents, U.S. Pat. No. 5,827,490 has issued recently. This patent, which is a continuation of an abandoned application, which in turn was a division of an abandoned divisional application of the above mentioned U.S. Pat. No. 5,281,403, suffers from some of the same deficiencies as are discussed immediately above. Furthermore, the ""490 Patent is generally drawn to arrangements wherein the reaction products of the urea hydrolyzation are maintained in the liquid phase, primarily for subsequent injection for SNCR applications. In one embodiment, the liquid is flashed to produce ammonia vapor for end uses requiring ammonia; however, because the teachings of the patent require maintaining the reaction products in a liquid phase, the ""490 system is not at all appropriate for producing gaseous ammonia on demand for large scale needs, such as SCR requirements. Pressure requirements for large volume production with the ""490 system are not practical, nor readily achievable.
The ""403 patent, as well as U.S. Pat. Nos. 4,168,299 and 5,252,308, were all used as anticipatory references in the International Search Report which was included with the recently published International Publication Number WO 98/42623. The ""623 International Application is cumulative of prior art discussed above, and is of no additional significance with respect to the subject invention. The ""623 International Application is drawn to a batch type once thru system. In FIG. 2, an overflow is illustrated; however this overflow is passive in nature (i.e. a measure to prevent over-fill conditions), and is not tied to process parameters, nor are measures addressed concerning rectifying problems when recirculating a solution containing ammonia suspended therein.
The basic parameters of a hydrolysis system for converting urea to ammonia has been known for many years. One of such prior systems can be seen in applications for handling waste solution in ammonia/urea manufacturing plants (see U.S. Pat. Nos. 5,399,755 and 4,235,816). In this regard, in such a manufacturing facility (which often costs in excess of $500 million dollars), natural gas is processed into Ammonia and CO2 in a first stage and thereafter, ammonia with the addition of CO2 is then further processed to produce urea. During a latter stage of the urea production, a waste stream of weak urea solution, and other contaminants, for which it is not appropriate to continue for the production of high quality urea, is passed through a hydrolyzing process and stripping tower for reduction of the contaminants in solution to a very low percentage (i.e. lower than 1 ppm), and that the xe2x80x9cpurifiedxe2x80x9d solution can be discharged or used as feed water. The hydrolyzer also produces ammonia and CO2, which may be reused in the production of urea. While this process is used successfully in many installations of urea production facilities, it does not in any manner describe or anticipate the invention herein, either alone or in reasonable combination. Indeed, this prior process is not a closed-loop system in the sense of the invention herein, nor does it anticipate the fact that the resultant weak solution can still retain a relatively substantial content of urea solution, nor is there any teaching therein of the utilization of a urea to ammonia hydrolysis process for the purpose of on-demand, on-site production of ammonia for SCR and FGC uses in fossil fuel burning facilities.